Fix broken vvalidation.
The engines created by triggers need to be set in ->engines.

All the tests are passing, some refactoring is still in order before being
able to merge this branch.
Remove a comment in the _build_engines method.
Move POD at the end and add empty sections for missing methods.
Remove useless debug in a test.
Move the triggers in the config to proper attributes.

There's two kinds of triggeres: on engines, and on config change.

We add a new attribute 'supported_engines' that contains a list of all the
supported engines by Dancer2.  In addition to that, two private attributes are
also added, one that contains a list of triggers associated to the engines,
and the second one is for the config change.
Clode cleanup for creating the engines.
Drop support for the key 'log' in config.

This was for compatibility reason in Dancer1, but this is not requried
in Dancer2.
Don't iterate over a manually created list of engines.

This list exists in the Config role with the attribute 'supported_engines'.